1. Field of the Invention
This invention relates to control systems for steam turbine generators, and more specifically, the methods and apparatus for precluding unnecessary tripping of the turbine in the event of transducer failure.
2. Prior Art
In a steam turbine generator system the turbine is normally maintained at constant speed and steam flow is varied to adjust the torque required to meet the electrical load imposed on the generator. This type of control is provided by the main control system which varies the flow of steam to the high-pressure turbine, and in some instances to the low-pressure turbine, to meet the load demand. The main control system is designed to accommodate for normal changes in load demand and to smoothly adjust the turbine operating conditions to the new demand. However, if the electrical load is suddenly lost or reduced significantly, a commensurate reduction must be made in the flow of steam through the turbine or the turbine will overspeed, possibly causing turbine damage. The main control system does not possess sufficiently rapid response characteristics to accommodate for such sharp variations in load demand, especially in the newer high power to inertia ratio turbine systems.
In addition to the main control system, the turbine generator combination is also provided with a protective or back-up control system which accommodates for abnormal changes in demand or failures in the main control system. A particularly effective protective system for dealing with load losses is disclosed in U.S. Pat. No. 3,643,437 (Birnbaum, et al.), which is assigned to the same assignee as this invention. That system relies on the fact that the power provided by the steam turbines, i.e. the input to the electrical generator, is approximately linearly related to the low-pressure turbine inlet steam pressure. Since most modern steam turbines are provided with reheat apparatus between the high-pressure and low-pressure turbines, this pressure is more commonly referred to as the reheat pressure. It follows then that, a comparison of the generator input and output can be made by comparing the reheat pressure with the electrical power provided by the generator. If a partial or total load loss of load occurs, the power provided by the generator will drop rapidly while the reheat pressure remains relatively constant, i.e., the power provided by the steam turbine to the electrical generator exceeds the electrical power generated.
In accordance with the Birnbaum system, the occurrence of a predetermined relationship between the reheat pressure and electrical output of the generator is interpreted as an indication of a partial load loss, and in response thereto the interceptor valve between the reheat apparatus and the low-pressure turbine is closed rapidly. This terminates the flow of steam to the low-pressure turbine while steam continues to be supplied to the high-pressure turbine. If this condition were permitted to continue, the relief valves on the reheat apparatus would open. However, according to the patent, the interceptor valve is only held closed momentarily, such as from 0.6 to 1.2 seconds, during which time additional load must be connected to the electrical generator. If the predetermined relationship between the reheat pressure and the electrical output still exists when the interceptor valve is reopened, the interceptor valve will again close and will continue to cycle until the condition is corrected or the operator takes other appropriate action.
Birnbaum utilizes the same relationship between the reheat pressure and the electrical output together with the additional indication that the main circuit breakers on the electrical generator are open, to detect a full load loss. Under these conditions both the governor valve, which controls the flow of steam to the high-pressure turbine, and the interceptor valve are closed to terminate all steam flow to the turbines. Resetting of the circuit breakers returns control to the main control system when the speed of the turbine drops below a preset value. If the breakers do not re-set, the governor valve and the interceptor valve will cycle until the turbine trips or corrective action is taken by the operator. Thus this system effectively and quickly responds to full load loss, yet also responds appropriately and without over-reaction to partial load losses.
The effective operation of any control system is the result of proper operation of, and interaction between, the various component parts of the system. Essential elements of a turbine system are the transducers which provide the inputs to the control system representative of the operating condition of various portions of the turbine system. Among these are the reheat pressure transducer and the wattmeter which provides a representation of the electrical output or load on the generator. It is evident that should one of these transducers fail, the failure could be detected by the protective system of Birnbaum as a partial load loss and the steam flow to the turbine could be interrupted repeatedly until the turbine was tripped. Such an unnecessary shutdown of the turbine is very costly.
Birnbaum attempts to provide for such a failure by preventing fast valving action when either the reheat pressure or the load signal is above the maximum or below the minimum value attainable in normal operation of the turbine. However, this does not provide for partial failure of a transducer such as when a pressure transducer shifts range. For instance, on the Hagen Model 109 Pressure Transducer used by the assignee of this invention to provide an input representative of the reheat pressure, it has been found that failure of a particular capacitor can cause the device to shift from one to five volts full range to two and one-half to five volts. Under these conditions the Birnbaum system would detect a mismatch indicative of a partial load loss, and since the transducer signal is not above the maximum or below the minimum, the interceptor valve would close. Since the mismatch would not be corrected by the fast valving, the interceptor valve would continue to cycle.
Similarly, the wattmeter used to measure the threephase electrical power provided by the generator can fail such that an erroneous signal between the minimum and maximum load signal is provided to the control system. For instance, the Hall wattmeter commonly used as the load transducer, utilizes the three currents and two of the voltages to determine the power delivered to the three-phase system. If one of the current signals is lost, such as by the blowing of a fuse, the power signal generated by the wattmeter will be only two-thirds of the actual power delivered. Again this is interpreted by the Birnbaum system as a partial load loss, causing continued cycling of the interceptor valve and eventual tripping of the turbine system.
In view of the foregoing, it is the primary object of this invention to provide more reliable steam turbine generator performance.
It is also an object of the invention to provide improved protection for the turbine system.
It is another object of the invention to preclude unnecessary tripping of the turbine system.